Project Summary/Abstract Despite a reduction in death rates attributed to respiratory distress syndrome and bronchopulmonary dysplasia in extremely premature infants over the past 20 years, death caused by infection has remained the same or has increased in rate. Specifically, respiratory viral infections contribute substantially to global fetal and infant losses and disproportionately affect preterm neonates. Of all deaths secondary to a respiratory viral infection, 55% occur in a neonate born before 30 weeks gestation. Over the past three years of a Career Development Award (K08) from the NIAID, Dr. Carey has developed the laboratory and leadership skills necessary for this application. Given the susceptibility of preterm infants to respiratory viruses, and the importance of CD8+ T cells in combating these pathogens, T-cell receptor (TCR) repertoire analysis was performed of nave CD8+ T cells in viable, live- born extremely preterm neonates (23-27 weeks gestation) and compared them to term neonates (37-41 weeks gestation), young children (16 months to 56 months) and adults (25 to 50 years old). Analysis revealed that preterm neonates have a repertoire characterized by shorter CDR3? regions, with a striking number of germline public clonotypes which exhibit convergent recombination. This degree of sharing indicates that there is a common, developmentally-determined ?beginner? repertoire, which is shaped by germline sequences. Therefore, we hypothesize that preterm infants are susceptible to infection because their nave T cell repertoire is dominated by germline, public TCR clones. This developmentally-determined beginner repertoire could provide a broad-based, cross-reactive protection, but not with the specificity and potency of an older child. Based on our preliminary work with a neonatal murine model of influenza infection and our comprehensive human nave CD8+ TCR repertoire analysis from the edges of viability to adulthood, we propose the following specific aims. First, perform nave CD4+ T cell repertoire analysis in neonates, infants and adults. Second, determine the TCR repertoire of infants infected with a respiratory virus. For this pilot study, we seek to enroll 30 infants in the two- year study period, with roughly 15 former preterm neonates and 15 former term neonates. CD8+ T cells and regulatory CD4+ T cells will be isolated and repertoire analysis performed. Little is known about the CD8+ T cell and Treg response during acute infection in the first year of life. Together, the results from this R03 pilot project will provide valuable information about the development of the complete nave T cell repertoire, how differences in the nave T cell repertoire might impact the response to a respiratory viral infection, and the role of Tregs in preterm versus term infants during infection. The results from this R03 study will provide key data to support a subsequent R01 submission. In this future R01 proposal, there will be an expanded depth of analysis of conventional CD4+, regulatory CD4+ and CTLs; the structure, function and transcriptional profile of these public clones will be explored using single cell RNA sequencing technology in a multi-center longitudinal proposal.